1. Field of the Invention
This invention relates to a power steering system including a flow control valve for the prevention of energy loss.
2. Description of Related Art
An example of power steering systems including a flow control valve for the prevention of energy loss is disclosed in Laid-open Japanese Patent Application No. 2001-260917 filed by the present applicant.
As shown in FIG. 4, the flow control valve V of the power steering system of the prior art example includes a spool 1 having an end adjoining a pilot chamber 2 and the other end adjoining another pilot chamber 3.
The pilot chamber 2 continuously communicates with a pump P via a pump port 4. Further, the pilot chamber 2 communicates via a flow path 6, a variable orifice a and a flow path 7 with an inflow port of a steering valve 9 provided for controlling a power cylinder 8.
The pilot chamber 3 incorporates a spring 5 and also communicates with the inflow port of the steering valve 9 via a flow path 10 and the flow path 7. Accordingly, the pilot chambers 2 and 3 communicate with each other via the variable orifice a, the flow path 7 and the flow path 10. Pressure upstream from the variable orifice a acts on the pilot chamber 2, and pressure downstream therefrom acts on the pilot chamber 3. The degree of opening of the variable orifice a is controlled by a solenoid current instruction value S1 for a solenoid SOL.
The spool 1 keeps a position at which the force acting on the pilot chamber 2, the force acting on the pilot chamber 3, and the force of the spring 5 are in balance. This balanced position determines the degree of opening of both the pump port 4 and tank port 11.
For example, upon actuation of a pump driving source 12 such as an engine or the like, the pump P is driven to supply pressure oil to the pump port 4 to cause a flow in the variable orifice a. This flow produces a pressure difference between the two sides of the variable orifice a, and the pressure difference causes a difference in pressure between the pilot chambers 2 and 3. The resultant differential pressure moves the spool 1 from the normal position, illustrated in FIG. 3, to the balanced position with opposing a force of the spring 5.
Thus, moving the spool 1 from the normal position toward the balanced position increases the degree of opening of the tank port 11. In accordance with the resultant degree of opening of the tank port 11, the distribution ratio between a control flow QP introduced toward the steering valve 9 from the pump P and a return flow QT circulating back to the tank T or the pump P is determined. In other words, the control flow QP is determined in accordance with the degree of opening of the tank port 11.
The control of the control flow QP in accordance with the degree of opening of the tank port 11 as described above results in determination of the control flow QP in accordance with the degree of opening of the variable orifice a. This is because the position to which the spool 1 is shifted and which determines the degree of opening of the tank port 11, is determined by the differential pressure between the two pilot chambers 2 and 3, and this differential pressure is determined by the degree of opening of the variable orifice a.
Thus, in order to control the control flow QP in accordance with vehicle speeds or steering conditions of the vehicle, the degree of opening of the variable orifice a, or the solenoid current instruction value SI for the solenoid SOL may be controlled. This is because the degree of the variable orifice a is controlled in proportion to an excitation current of the solenoid SOL so that the variable orifice a holds the degree of its opening to a minimum in a non-excited state of the solenoid SOL and increases the degree of its opening as the excitation current is increased.
The steering valve 9 applied with the control flow QP controls the amount of oil supplied to the power cylinder 8 in accordance with input torque (steering torque) of the steering wheel (not shown). For example, if the steering torque is large, the amount of shifting of the steering valve 9 is increased to increase the amount of oil supplied to the power cylinder 8, whereas if it is small, the amount of shifting of the steering valve 9 is decreased to decrease the amount of oil supplied to the power cylinder 8. The higher amount the pressure oil is supplied, the higher assist force the power cylinder 8 exerts, and the smaller amount the pressure oil is supplied, the lower assist force the power cylinder 8 exerts.
It should be noted that the steering torque and the amount of shifting of the steering valve 9 are determined by a torsion reaction of a torsion bar (not shown) or the like.
As described above, the steering valve 9 controls a flow QM supplied to the power cylinder 8 and the flow control valve V controls the control flow QP supplied to the steering valve 9. If the flow QM required by the power cylinder 8 comes as close as possible to the control flow QP determined by the flow control valve V, it is possible to reduce the energy loss around the pump P. This is because the energy loss around the pump P is caused by a difference between the control flow QP and the flow QM required by the power cylinder 8.
In order to make the control flow QP as close as possible to the flow QM required by the power cylinder 8 for the prevention of energy loss, the system of the prior art example controls the degree of opening of the variable orifice a. The degree of opening of the variable orifice a is determined by the solenoid current instruction value SI for the solenoid SOL as described earlier. A controller C, described below in detail, controls the solenoid current instruction value SI.
The controller C is connected to a steering angle sensor 14 and a vehicle speed sensor 15. As illustrated in FIG. 5, the controller C determines a current instruction value I1 based on a steering angle detected by the steering angle sensor 14, and also a current instruction value I2 based on a steering angular velocity calculated by differentiating the steering angle.
The relationship between the steering angle and the current instruction value I1 is determined on the basis of theoretical values giving linear characteristics to the relationship between the steering angle and the control flow QP. The relationship between the steering angular velocity and the current instruction value I2 is also determined on the basis of the theoretical values giving linear characteristics to the relationship between the steering angular velocity and the control flow QP. Both of the current instruction values I1 and I2 are outputted at zero unless the steering angle and the steering angular velocity exceed a set value. Specifically, when the steering wheel is positioned at or around the center, the current instruction values I1 and I2 are outputted at zero in order to set a dead zone around the center.
Further, the controller C outputs a steering angle-dedicated current instruction value I3 and a steering angular velocity-dedicated current instruction value I4 on the basis of a detected value provided by the vehicle speed sensor 15.
The steering angle-dedicated current instruction value I3 is outputted at 1 at low vehicle speeds and, for example, at 0.6 at highest vehicle speeds. In addition, the steering angular velocity-dedicated current instruction value I4 is outputted at 1 at low vehicle speeds and, for example, at 0.8 at highest vehicle speeds. In other words, in from low vehicle speeds to highest vehicle speeds, a gain for the steering angle-dedicated current instruction value I3 controlled in the range of 1 to 0.6 is set to be larger than that for the steering angular velocity-dedicated current instruction value I4 controlled in the range of 1 to 0.8.
The steering angle-dedicated current instruction value I3 is then multiplied by the current instruction value I1 based on the steering angle. Because of this, the steering angle-based current instruction value I5 resulting from this multiplication becomes smaller as the vehicle speed increases. In addition, the gain for the steering angle-dedicated current instruction value I3 is set to be larger than that set for the steering angular velocity-dedicated current instruction value I4. Therefore, the faster the vehicle speed becomes, the higher the rate of decrease of the value I5 becomes. That is to say, the response is kept high at low vehicle speeds and is reduced at high vehicle speeds. The reason for changing in speed of the response in accordance with the vehicle speeds is that the high response is not usually required during high-speed travel but is necessary, in most of the cases, at low vehicle speeds.
The controller C provides the current instruction value I2 based on the steering angular velocity with the steering angular velocity current instruction value I4 as a limit value and outputs a steering angular velocity-based current instruction value I6. This current instruction value I6 is also reduced in accordance with the vehicle speeds. However, the gain for the steering angular velocity-dedicated current instruction value I4 is set to be smaller than that set for the steering angle-dedicated current instruction value I3. Therefore, the reducing rate for the current instruction value I6 is smaller than that for the current instruction value I5.
The reason for setting the limit values in accordance with vehicle speeds is mainly for the prevention of an excessive assist force from exerting during high-speed travel.
The steering angle-based current instruction value I5 and the steering angular velocity-based current instruction value I6 outputted as described above are compared and the larger value of the two is adopted.
For example, the steering wheel is rarely rotated abruptly during high-speed travel, and therefore it is common that a steering angle-based current instruction value I5 is larger than a steering angular velocity-based current instruction value I6. Because of this, in most cases, the steering angle-based current instruction value I5 is selected during high-speed travel. In order to enhance the safety and stability of handling the steering wheel at this time, the gain for the current instruction value I5 is set to be larger. In other words, as the travelling speed becomes faster, the proportion for making the control flow QP small becomes larger for enhancement of the safety and stability during travel.
On the other hand, the steering wheel is often rotated abruptly during travel at low vehicle speeds, so that, in many cases, the value of the steering angular velocity becomes larger than that of the steering angle. Because of this, during low-speed travel, the steering angular velocity-based current instruction value I6 is selected in most cases. When the steering angular velocity is larger, the response is regarded to be of importance.
Accordingly, during travel at low vehicle speeds, the gain for the steering angular velocity-based current instruction value I6 maintains small with reference to the steering angular velocity in order to enhance the response, that is, the handling of the steering wheel. In other words, when the traveling speed becomes faster for some extent, even if the steering wheel is abruptly rotated, the response is secured by means of obtaining adequate control flow QP.
The current instruction value I5 or I6 selected as described above is added to a standby current instruction value I7. The resultant value of the addition of the standby current instruction value I7 is outputted to a driver 16 as a solenoid current instruction value SI.
Due to the addition of the standby current instruction value I7, the solenoid current instruction value SI is kept to be the predetermined magnitude even when all the current instruction values based on the steering angles, the steering angular velocities and the vehicle speeds are at zero. For this reason, a predetermined oil flow is supplied to the steering valve 9 at all times. In view of the prevention of energy loss, it is ideal for the control flow QP in the flow control valve V to be zero when the flow QM required by the power cylinder 8 and the steering valve 9 is zero. In other words, reducing the control flow QP to zero means causing the total amount of oil discharged from the pump P to return back to the pump P or the tank T from the tank port 11. The path of oil flow returning from the tank port 11 to the pump P or the tank T is extremely short in the main body B, so that little pressure loss occurs. Due to little pressure loss, driving torque of the pump P is lessened to a minimum, resulting in energy conservation. In this context, with regard to the prevention of energy loss, it is advantageous for the control flow QP to be zero when the required flow QM is zero.
Nevertheless, a standby flow QS is maintained even when the required flow QM is zero. This is because of the following.
(1) To prevent seizure in the system. The circulation of the standby flow QS through the system can provide cooling effects.
(2) To ensure response. The maintenance of the standby flow QS as described above results in a reduction of the time required for attaining a target control flow QP as compared with the case of no maintenance of the standby flow QS. The resultant time difference affects the response. As a result, the maintenance of the standby flow QS leads to improvement in the response.
(3) To counter disturbances, such as kickback and the like, and self-aligning torque. When reaction to disturbance or self-aligning torque acts on the wheels, the reaction acts on the rod of the power cylinder 8. If the standby flow is not maintained, the reaction to the disturbance or self-aligning torque makes the wheels unsteady. However, the maintenance of the standby flow prevents the wheels from becoming unsteady even when the reaction as described above acts on the wheels. Specifically, the rod of the power cylinder 8 engages with a pinion and the like for switching the steering valve 9. Hence, upon the action of the reaction, the steering valve is also switched to supply the standby flow in a direction counter to the reaction. Therefore, maintaining the standby flow makes it possible to counter the self-aligning torque and the disturbance caused by kickback.
Next, a description will be given of the operation of the power steering system of the prior art example.
During travel of a vehicle, the controller C outputs a steering angle-based current instruction value I5 which is the resultant value of multiplication of a solenoid current instruction value I1 based on a steering angle by a steering angle-dedicated current instruction value I3, and also outputs a steering angular velocity-based current instruction value I6. The current instruction value I6 is determined by a solenoid current instruction value I2, based on the steering angular velocity, that is limited by a steering angular velocity-dedicated current instruction value I4 serving as a limit value.
Next, the steering angle-based current instruction value I5 and the steering angular velocity-based current instruction value I6 are compared with each other and the larger value of the two current instruction value I5, I6 is added to a standby current instruction value I7 to determine a solenoid current instruction value SI at this point. The solenoid current instruction value SI is provided mainly in reference to the steering angle base current instruction value I5 during high-speed travel of the vehicle and to the steering angular velocity base current instruction value I6 during low-speed travel of the vehicle.
It should be noted that the spool 1 has a slit 13 formed on its leading end. The slit 13 allows the pilot chamber 2 to communicate with the variable orifice a even when the spool 1 is at the normal position as illustrated in FIG. 4. Specifically, even when the spool 1 is in the normal position, the pressure oil having supplied from the pump port 4 to the pilot chamber 2 is supplied to the steering valve 9 via in order of the slit 13, the flow path 6, the variable orifice a, and the flow path 7, thereby providing with the prevention of seizure of the system, and disturbance such as kickback or the like, and the provision of the response.
FIG. 4 illustrates a driver 16 provided for driving the solenoid SOL and connected to the controller C and the solenoid SOL, throttles 17 and 18, and a relief valve 19.
The prior art power steering system as described above determines the solenoid current instruction value SI mainly with reference to the steering angle-based current instruction value I5 during high-speed travel and mainly with reference to the steering angular velocity-based current instruction value I6 during low-speed travel.
However, the solenoid current instruction value SI may be determined with reference to the steering angular velocity-based current instruction value I6 even during high-speed travel. In such a case, with regard to the stability and the safety, it is desired that the sensitivity is decreased with reducing the response of the current instruction value I6.
However, the prior art power steering system is incapable of slowing down the response of the current instruction value I6 because the steering angular velocity-based current instruction value I2 is not multiplied by the value of the gain in accordance with the vehicle speeds. Therefore, there is a problem that a driver feels uncomfortable during high-speed travel because the response is too high.